In terrestrial-based cellular systems, channel assignments are made within fixed frequency reuse cell clusters. A cluster comprises a set of predetermined cells which are adjacent to each other. That is, each cell within a predetermined cluster is given a unique set of orthogonal channels so that channels within a cluster do not interfere with one another. Outside of the cluster, the channels are reused. The reuse between clusters follows a fixed pattern designed to minimize interference between the clusters.
The reuse cluster technique associated with terrestrial-based cellular systems is difficult to implement when the cellular base stations are replaced by satellites in a low-earth orbit (LEO). Time division multiplex access (TDMA) and frequency division multiple access (FDMA) communication systems and systems that use similar channel structures avoid excessive interference by assigning traffic channels that do not conflict with other traffic channels in time or frequency. The channel assignment problem is exacerbated when one end of the communication system is located at the LEO satellites where differential Doppler shifts and differential propagation times can cause transmissions between a user and a satellite to interfere with channels on another satellite. This problem becomes even more severe when earth terminals or stations can transmit and receive over large coverage angles that may include several satellites.
Accordingly, there is a significant need for methods which efficiently use the available frequency spectrum without interfering with other channels.